Laminates and multi-ply plastic and glass composites having at least one lamina or layer of a sensitive thermoplastic resin, such as, polycarbonate are well known and commercially available. In particular, thermoplastic laminae or layers bring to these laminates or composites such properties as high heat resistance, dimensional stability, and especially, high impact strength. These properties render the thermoplastic containing composites amenable to a wide variety of uses, such as for example, aircraft windows, automobile safety glass, or bullet resistant glass.
In such laminates or composites, a thermoplastic resinous lamina or layer is often adhered to a second lamina or layer of for example, polycarbonate, polyacrylate, or glass by means of an adhesive interlayer which may contain additives. Adhesive interlayers are prepared from various synthetic polymers, such as for example, polyvinyl butyral, polyurethane, poly(ethylene-vinylacetate), poly (ethylene-vinylacetate-vinylalcohol), poly (ethylene-methylmethacrylate-acrylic acid), etc.
Unfortunately, many thermoplastic resins are somewhat susceptible to stress cracking and crazing in the presence of a substantial number of organic compounds, particularly many of the organic compounds used as additives in adhesive interlayers. Additives, particularly plasticizers, of the adhesive interlayer often migrate to the interlayer/resin interface and enter the thermoplastic resin resulting in stress cracking, crazing, loss of transparency, and/or loss of adhesion as well as undesirable changes to the physical properties of the thermoplastic resin,. The foregoing is particularly applicable to the widely used polycarbonate resins.
U.S. Pat. No. 3,269,971 assigned to the same assignee at the present invention, discloses the adipates, benzoates, chlorinated hydrocarbons, epoxy compounds, glycerol derivatives, glycolates, phosphates, phosphites, phosphonates, phthalates, etc., as unsatisfactory plasticizers for polycarbonate and discloses instead, the use of the carbonate esters. Although the carbonate esters may be compatible with the polycarbonates, they may not be suitable plasticizers for all adhesive interlayers and regardless of compatibility, their migration into the polycarbonate may injure essential properties of the system.
Laminates containing polycarbonate sheets in contact with plasticized polyvinyl acetal sheets are described in U.S. Pat. Nos. 3,406,086 and 3,539,442. Disclosed plasticizers for the polyvinyl butyral layers are phosphates and sulfonamides respectively and these work very well giving rise to few problems. However, the use of these plasticizers requires that the polyvinyl acetal resin be cast to form a sheet rather than extruded. Extrusion is more efficient to produce large volumes of sheet, but the high temperature ranges used to extrude polyvinyl acetal decompose the recommended plasticizers.
U.S. Pat. No. 4,027,069 discloses a laminated safety glass having at least one layer of glass laminated to at least one layer of plasticized and buffered polyvinyl acetal sheet. A third layer also laminated to the plasticized and buffered polyvinyl acetal sheet may optionally be polycarbonate. However, there is no disclosure of a means to prevent the migration of the plasticizer into the polycarbonate and thereby prevent stress cracking and crazing.
U.S. Pat. No. 4,128,694 discloses a sheet of polycarbonate in face-to-face contact with a sheet of polyvinyl butyral plasticized with an effective amount of plasticizer which plasticizer is at least in part an ester formed by reaction of an alcohol having from 2 to 4 hydroxyl groups and a C.sub.16 -C.sub.20 unsaturated fatty acid having a hydroxyl group attached to the acid molecule. This invention suffers from the severe limitation on the plasticizer that may be used in the adhesive interlayer.
U.S. Pat. No. 4,328,277, issued to the same assignee as the present invention, describes a polycarbonate back lamina having an external mar-resistant coat as well as an optional internal mar resistant coat, which internal side of the lamina is the side of adherence to an adhesive interlayer. The stated purpose of the mar-resistant coat is to prevent marring during lay-up and the like. This reference does not disclose the use of plasticizers in the adhesive interlayer and furthermore, specifically requires that the interlayer be chemically compatible with the sensitive polycarbonate.
Thus, though use of thermoplastic resins such as polycarbonate in laminates and multi-ply composites can substantially improve shatter resistance, strength, and weight, the use of these resins is severely restricted by incompatibility with the common plasticizers of adjacent layers, in particular, plasticized adhesive interlayers such as polyvinyl butyral adhesive interlayers.
Therefore, it is an object of the present invention to provide a method for preventing the stress crazing and cracking of sensitive thermoplastic resinous laminae or layers, such as, a polycarbonate lamina or layer by incompatible additives of adjacent laminae or layers.
It is another object of the present invention to provide a laminate or multi-layer composite having at least one lamina or layer of a sensitive thermoplastic resin, such as, polycarbonate resin coated with a cross-linked protective coat in face-to-face contact with a plastic lamina or layer containing incompatible additives.
It is yet another object of the present invention to provide sensitive resinous laminae or layers with a single protective coating which protects the interior surface from incompatible adjoining laminae or layers yet which also protects or may become a component of a coating system which protects the external surface from scratch and abrasion.